1. Field of the Invention
The invention relates generally to pulsed neutron well logging instruments. More specifically, the invention relates to neutron burst timing sequences used to acquire selected types of pulsed neutron measurements to determine selected properties of earth formations.
2. Background Art
Pulsed neutron well logging instruments known in the art are used to measure a number of different properties of earth formations penetrated by wellbores. The most common types of pulsed neutron instruments include an accelerator-type source which emits controlled duration xe2x80x9cburstsxe2x80x9d of high energy neutrons into the formations surrounding the wellbore, and one or more detectors that measure numbers of neutrons, particularly epithermal energy and thermal energy, as well as gamma rays which are emitted as a result of the interaction of the neutrons with the formations surrounding the wellbore and the fluids in the wellbore itself. The gamma rays may include inelastic gamma rays which are emitted by high-energy collisions of the neutrons with atomic nuclei in the earth formations, as well as capture gamma rays emitted when low energy (thermal) neutrons are captured by susceptible atomic nuclei in the formations. Properties of the earth formations which may be determined as a result of measuring neutron detection rate and gamma rays include bulk density of the formation, fractional volume of void or pore space in the formation (porosity), and neutron capture cross section, among other measurements. Properties which may be determined by spectral analysis of the gamma rays include concentration of various chemical elements, for example. Properties of fluids in the wellbore may also be determined from various neutron and gamma ray measurements.
Instruments which can make measurements of a plurality of the foregoing types are described, for example, in U.S. Pat. No. 6,032,102 issued to Wijeyesekera et al., and in U.S. Pat. No. Re. 36,012 issued to Loomis et al., both of which are assigned to the assignee of the present invention. Generally speaking, the instruments disclosed in these patents are arranged so that a pulsed neutron source therein emits a plurality of short duration neutron bursts, these being of a duration to enable relatively accurate measurement of density, through spectral analysis of inelastic gamma rays, and accurate measurement of porosity, through measurement of neutron xe2x80x9cslowing down timexe2x80x9d, or rate of decay of detected neutron count rate with respect to time shortly after the end of each neutron burst. A neutron detector positioned axially proximate the source is used on such instruments to make the neutron slowing down time measurements. A gamma ray detector positioned appropriately with respect to the source, and coupled to a spectral analyzer, is used to make the inelastic gamma ray measurements. The short duration bursts are repeated for a selected number of times and the measurements made in appropriate time windows during and/or after each neutron burst are summed or stacked to improve the statistical precision of the measurements made therefrom.
The instruments described in the above patents are also adapted to measure neutron capture cross section of the earth formations. The manner in which these instruments make neutron capture cross section measurements is conventional, and includes, after the previously described plurality of short duration bursts and measurement windows, a relatively long duration burst, after which capture gamma rays or thermal neutrons are measured at appropriately spaced detectors therefor. The measurements are characterized to determine neutron capture cross section of the formations surrounding the instrument.
Another pulsed neutron instrument which makes measurements related to neutron interaction phenomena induced by both short duration and long duration neutron bursts is described, for example, in U.S. Pat. No. 4,926,044 issued to Wraight. Generally, this instrument makes measurements, after short duration neutron bursts, of phenomena related to short capture cross section components in the wellbore and formations surrounding the instrument. Longer capture cross section components are evaluated by operating radiation detectors after long duration bursts, as is conventional for capture cross section determination.
A pulsed neutron logging instrument which is adapted to measure both xe2x80x9cshort burstxe2x80x9d-related neutron interaction phenomena, such as density and porosity, as well as xe2x80x9clong burstxe2x80x9d-related neutron interaction phenomena, such as capture cross section, has an inherent limitation. This limitation is that the long bursts used to make their associated measurements reduce the number of available neutrons for making measurements associated with the short bursts. While a burst timing sequence could be devised which increases the number of short burst measurement cycles at the expense of the number of long burst measurement cycles, such a timing sequence may reduce the statistical precision of the long burst related measurements, or may limit the speed at which the logging instrument may be moved along the wellbore while still making reasonably precise measurements.
It is desirable to have a pulsed neutron well logging instrument and method which can make both long burst related measurements and short burst related measurements with improved statistical precision.
One aspect of the invention is a method for measuring neutron interaction properties of an earth formation. The method includes irradiating the formation with bursts of high energy neutrons. The bursts have a duration selected to enable detection of short duration neutron burst related phenomena. During or after at least one of the bursts, short burst related phenomena are detected. After a selected number of the bursts, long duration neutron burst-related phenomena are detected.
In some embodiments, the short duration burst related phenomena include at least one of the following types: neutron slowing down time related phenomena, inelastic gamma ray related phenomena, and short-duration capture cross section related phenomena. In some embodiments, a length of time during which the long duration neutron burst phenomena are detected is selected to optimize a duty cycle of a pulsed neutron source used to perform the activating. In some embodiments, the length of time is selected so as to optimize the duty cycle and the statistical precision of measurements of capture cross section at a highest expected value thereof for the earth formations being evaluated. In some embodiments, the length of time is selected to optimize accuracy of measurement of the long duration burst phenomena.
Another aspect of the invention is an instrument for detecting neutron interaction phenomena in earth formations surrounding a wellbore. The instrument includes a controllable source of high energy neutrons. The instrument includes radiation detectors. At least one of the detectors is adapted to detect neutron slowing down phenomena occurring in the earth formations and at least one of the detectors is adapted to detect inelastic gamma ray phenomena. At least one of the detectors is adapted to detect long duration neutron burst related phenomena. The instrument includes a controller operatively coupled to the source and detectors. The controller is adapted to cause the neutron source to emit bursts of high energy neutrons. The bursts have a duration selected to enable detection of the inelastic gamma ray phenomena. The controller is adapted to cause detection of the inelastic gamma ray phenomena during each burst and to cause detection of the neutron slowing down time phenomena proximate the end of each burst. The controller is adapted to cause detection of the capture cross section phenomena after the end of a selected number of the bursts.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.